1. Field of the Invention
The present invention relates to a monocomponent type developing apparatus which is applicable for example to copying machines, printers, and facsimile apparatuses for forming an image by electrophotography, specifically, to a reversal-development type developing apparatus for performing reversal development of an electrostatic latent image by causing a toner support member to contact to an electrostatic latent image support member, while causing a developing bias source to apply a developing bias voltage to the toner support member, and supplying charged toner held by the toner support member to the electrostatic latent image support member.
2. Description of the Related Art
Conventionally, in a dry developing apparatus for developing an electrostatic latent image of an electrophotographic-type image forming apparatus is widely used a two-component developing method using a developer composed of toner and carrier. A typical two-component developing method is a magnetic-brush developing method of erecting the carrier by magnetic field generating means and performing development with the use of toner held on the surface of the carrier.
Although this magnetic-brush developing method using toner and carrier is sufficiently practical, a monocomponent developing method has been widely studied recently, because a downsized development apparatus is increasingly required and maintenance is facilitated in a monocomponent developer which is composed of toner only.
An example of a developing apparatus using a monocomponent developer is shown in FIG. 5. The developing apparatus shown in FIG. 5 is equipped with a photoreceptor 51 serving as an electrostatic latent image support member, a developing roller 52 serving as a toner support member which is disposed so as to face to the photoreceptor 51 and capable of conveying toner while the toner deposits onto a surface thereof, a developer control member 54 for controlling toner held on the developing roller 52 and forming the toner into a thin layer, a toner supply roller 53 for supplying toner to the developing roller 52, a case 55 for storing toner 56 to be supplied to the developing roller 52, a developing bias source 57 for applying a developing bias voltage to the developing roller 52, and a supplying bias source for applying a supplying bias voltage to the toner supply roller.
The toner supply roller 53 rotates in contact with or in proximity to the developing roller 52, whereby the toner 56 stored in the case 55 is stably supplied to the developing roller 52 and an even and thin layer is formed by the developer control member 54. The thin-layered toner 56 is conveyed by a rotation of the developing roller 52 to a developing region where the developing roller 52 and the photoreceptor 51 contact and face to each other, and in this developing region, the toner 56 transfers to an electrostatic latent image on the photoreceptor 51, whereby the image is visualized.
In this monocomponent type developing apparatus, in order to increase a performance in toner supply, it is better to apply a bias voltage to the developing roller 52 and the toner supply roller 53, respectively, which bias voltage generates an electric field in a direction that the toner 56 is supplied from the toner supply roller 53 to the developing roller 52, than to apply a bias voltage of the same potential with the developing roller 52 to the toner supply roller 53. In other words, when the toner is positively charged, an excellent performance in toner supply is obtainable by causing the supplying bias voltage applied to the toner supply roller 53 to be larger than the developing bias voltage applied to the developing roller 52. On the contrary, when the toner is negatively charged, an excellent performance in toner supply is obtainable by causing the developing bias voltage to be larger than the supplying bias voltage.
In the above-mentioned monocomponent type developing apparatus, when the developing apparatus is activated in a state that the developing roller 52 holding toner is halted in contact with the photoreceptor 51 as further shown in FIG. 6, a charger 61 is turned on at the same time that the developing roller 52 and the photoreceptor 51 are rotated, whereby the photoreceptor 51 is uniformly charged by the charger 61. After that, a part which is irradiated by a not-shown exposing device such as LED is discharged and then reversal development of adhering toner to this part is performed.
In this case, on the photoreceptor 51, an uncharged region X is not charged by the charger 61, which uncharged region is formed between the charger 61 and a place where the photoreceptor contacts the developing roller 52 on the downstream side of a direction that the photoreceptor rotates, with the result that a surface potential thereof remains 0 V, that is, uncharged. Even when the developing bias source 57 is turned off and the potential of the developing roller 52 is kept to be 0 V for a time period before the uncharged region X contacts and passes by the developing roller 52, toner held on the developing roller 52 is charged either positively or negatively, so that a potential difference is generated between the toner and the photoreceptor 51 and the toner is likely to deposit on the photoreceptor 51. Furthermore, there arises a problem that, due to a mechanical friction between the toner and the photoreceptor 51, a large quantity of charged toner deposits on the uncharged region X and the toner is wasted.
As means for avoiding the above-said problems, Japanese Unexamined Patent Publication JP-A 4-57079 (1992) discloses, as shown in FIG. 7, a configuration that a Zener diode 71 is provided as a constant-voltage element between the developing roller 52 and the developing bias source 57 in a direction to block a flow of electric charge induced by the developing roller 52, a polarity of which electric charge is opposite to that of toner.
FIG. 7 shows a developing apparatus which uses negatively charged toner. In a case where the toner is consumed in the uncharged region X on the photoreceptor when development is started, positively charged electric charge whose polarity is opposite to that of the toner is induced by the consumed amount. A flow of the induced positive electric charge in a direction to the developing bias source 57 is blocked by the Zener diode 71.
According to the above-described configuration, the developing roller 52 is kept in an electrically floating state up to a voltage of causing a Zener breakdown, and the positive electric charge is held by the developing roller 52. Therefore, the negatively charged toner is attracted to the developing roller 52 and the toner is not supplied from the developing roller 52 to the uncharged region X on the photoreceptor 51, whereby a waste of toner is restricted.
As described above, according to the configuration that the Zener diode 71 is provided between the developing roller 52 and the developing bias source 57, the developing roller 52 is kept in an electrically floating state up to a voltage of causing a breakdown of the Zener diode 71. However, in FIGS. 6 to 8, there arises a problem that the developing bias voltage varies in a range up to the voltage of causing a breakdown of the Zener diode 71 when reversal development is performed.
Further, JP-A 4-57079 discloses a configuration that, in order to solve the above-said problem, conducting means 83 is provided as shown in FIG. 8 for causing the Zener diode 71 to break down and to be in a conducting state when the developing bias voltage is applied, whereby a stable developing bias voltage can be obtained even when reversal development is performed.
However, in either of the above configurations, it is necessary to use a Zener diode of a high breakdown voltage, in order to reliably realize the floating state.
Further, in a case where the Zener diode 71 is kept in a conducting state by the conducting means 83 as shown in FIG. 8, the sum of a difference of potentials given to both ends of a protective resistance 82 which forms the conducting means 83 and a forward voltage of a diode 81 is regarded as the developing bias voltage.
Therefore, there is a problem that, in a case where it is necessary to set the developing bias voltage to be high for the purpose of, for example, increasing an image density, a resistance of a high resistance to pressure is needed as the protective resistance 82. Furthermore, since the Zener diode 71 of a high breakdown voltage is used in order to reliably realize the floating state, there is a problem that a source with a considerably higher voltage than the developing bias voltage is inevitably needed as the developing bias source 57.